In recent years, alkaline storage batteries, especially nickel-hydrogen storage batteries, have been used for an electric current source for vehicles such as HEVs or PEVs. Long-term durability performance such as high output performance far beyond the conventional level and self-discharge characteristics is required in alkaline storage batteries for these types of use. Therefore, as a technique for higher output, increasing a reaction area between a positive electrode and a negative electrode as shown in JP-A-2000-082491 and JP-A-2007-294219 has been proposed.
In JP-A-2000-082491, in which increasing a reaction area is proposed, a positive electrode area is made to be 30 cm2 or more per theoretical capacity (Ah) of a battery. This is based on the idea that the wider reaction area between a positive electrode and a negative electrode in a contained electrode group reduces the density of a current flowing between both electrodes, leading to no increase in the internal resistance of the electrode group when operating the battery at a high discharge rate; therefore, a high discharge current can be obtained without any decrease in an operating voltage. In this case, a value of the above-mentioned positive electrode area less than 30 cm2/Ah does not reduce the internal resistance of the electrode group, leading to a decrease in an operating voltage and difficulty in attaining a high current discharge.
Meanwhile, the surface area of a negative electrode is made to be 120 cm2 or more per nominal battery capacity (Ah) for increasing a reaction area that the inventors of the present invention proposed in JP-A-2007-294219.
However, even if a reaction area is increased as proposed in the above-mentioned JP-A-2000-082491 and JP-A-2007-294219, the following other two problems emerge.
A first problem is that there is a region in which no improvement of output characteristics is recognized even after increasing a reaction area. This means that increasing the number of electrode plate layers as an electrode group to increase the reaction area prevents an electrolyte from prevailing in the whole electrode plate, leading to a concentration of the electrolyte to both terminals in the direction of the short axis of a negative electrode plate as well as increase in a mass of the electrolyte stored in a separator. This reduces an actual reaction area of the electrode plate, leading to no improvement of output characteristics. In this case, increasing an electrolyte mass is shown to provide no resolution.
A second problem is that using an AB5 type hydrogen storage alloy used in general as a negative electrode active material as a negative electrode plate with an increased reaction area is found to cause a degradation of self-discharge characteristics (an increase in self-discharge). This is partly because the distance between positive and negative electrode plates is shortened due to increasing the reaction area. Along with this, adding manganese and cobalt is essential to maintain the crystal structure of the AB5 type hydrogen storage alloy.
Then, when the AB5 type hydrogen storage alloy to which manganese and cobalt are added is oxidized, the added manganese and cobalt are eluted and deposited. This accelerates self-discharge, leading to degradation of self-discharge characteristics.
In this case, increasing an electrolyte mass retained in the negative electrode plate to solve the first problem causes further elution of manganese and cobalt, leading to marked degradation of self-discharge characteristics.